refactor Region, part 4 - group compatibility functions

- new .igs property
- update .update_shape(), .get_vertices_of_cells(), .get_vertices(),
  .get_edges(), .get_faces(), .get_surface_entities() -> .get_facets(),
  .get_cells()
- remove .delete_zero_faces() body, .select_cells(),
  .select_cells_of_surface(), .has_cells_if_can()
- update docstring